Quantum dots have a semiconductor nanostructure that binds the conduction band electrons, valence band holes and excitons in three spatial directions. For example, the quantum dots may be nanoparticles composed of elements IIB and VIA or elements IIIA to VA. The particle size of the quantum dots generally ranges from about 1 nm to about 10 nm. Due to quantum confinement of the electrons and holes, the continuous band structure becomes a discrete energy level structure with molecular characteristics, and the quantum dots can be excited to emit fluorescence. Based on a quantum effect, quantum dots have wide potential applications in the fields such as solar cells, light-emitting devices and optical biomarkers. Scientists have invented many different ways to fabricate quantum dots, and such nanomaterials are expected to have great potential in the application of nano-electronics in twenty-first century.
The quantum dot electroluminescent devices mainly have two excitation mechanisms: one is direct injection of carriers, that is, injection of holes from a hole transport layer to a quantum dot light-emitting layer, and injection of electrons from an electron transport layer to the quantum dot light-emitting layer, and combining the holes and the electrons in the quantum dot light-emitting layer to form excitons and emit light; the other is by means of energy transfer, that is, direct transfer of the energy from excitons formed in the transport layer to the quantum dots results in emitting light.
A typical quantum dot light-emitting diode structure comprises a hole injection layer, a hole transport layer, an electron transport layer, and a quantum dot light-emitting layer, and the materials of the electron transport layer, the hole transport layer and the hole injection layer may be organic small molecules, organic polymers or inorganic metal oxides. However, because the valence band of the quantum dots is far below the highest occupied molecular orbital (HOMO) level of the hole transport layer, the injection ability of the holes is poor, which leads to the imbalance of the carriers and further leads to current leakage and degradation of devices, and thus the light-emitting efficiency and the lifetime of the devices are affected.